The disclosure concerns a method and apparatus for heating materials by application of radio frequency (“RF”) energy, also known as electromagnetic energy. In particular, the disclosure concerns an advantageous method for RF heating of materials with a low or zero electric dissipation factor, magnetic dissipation factor, and electrical conductivity, such as petroleum ore. For example, the disclosure enables efficient, low-cost heating of bituminous ore, oil sands, oil shale, tar sands, or heavy oil.
Bituminous ore, oil sands, tar sands, and heavy oil are typically found as naturally occurring mixtures of sand or clay and dense and viscous petroleum. Recently, due to depletion of the world's oil reserves, higher oil prices, and increases in demand, efforts have been made to extract and refine these types of petroleum ore as an alternative petroleum source. Because of the extremely high viscosity of bituminous ore, oil sands, oil shale, tar sands, and heavy oil, however, the drilling and refinement methods used in extracting standard crude oil are typically not available. Therefore, bituminous ore, oil sands, oil shale, tar sands, and heavy oil are typically extracted by strip mining, or in situ techniques are used to reduce the viscosity by injecting steam or solvents in a well so that the material can be pumped. Under either approach, however, the material extracted from these deposits can be a viscous, solid or semisolid form that does not easily flow at normal oil pipeline temperatures, making it difficult to transport to market and expensive to process into gasoline, diesel fuel, and other products. Typically, the material is prepared for transport by adding hot water and caustic soda (NaOH) to the sand, which produces a slurry that can be piped to the extraction plant, where it is agitated and crude bitumen oil froth is skimmed from the top. In addition, the material is typically processed with heat to separate oil sands, oil shale, tar sands, or heavy oil into more viscous bitumen crude oil, and to distill, crack, or refine the bitumen crude oil into usable petroleum products.
The conventional methods of heating bituminous ore, oil sands, tar sands, and heavy oil suffer from numerous drawbacks. For example, the conventional methods typically utilize large amounts of water, and also large amounts of energy. Moreover, using conventional methods, it has been difficult to achieve uniform and rapid heating, which has limited successful processing of bituminous ore, oil sands, oil shale, tar sands, and heavy oil. It can be desirable, both for environmental reasons and efficiency/cost reasons to reduce or eliminate the amount of water used in processing bituminous ore, oil sands, oil shale, tar sands, and heavy oil, and also provide a method of heating that is efficient and environmentally friendly, which is suitable for post-excavation processing of the bitumen, oil sands, oil shale, tar sands, and heavy oil.
One potential alternative heating method is RF heating. “RF” is most broadly defined here to include any portion of the electromagnetic spectrum having a longer wavelength than visible light. Wikipedia provides a definition of “radio frequency” as comprehending the range of from 3 Hz to 300 GHz, and defines the following sub ranges of frequencies:
NameSymbolFrequencyWavelengthExtremely lowELF3-30 Hz10,000-100,000 kmfrequencySuper low frequencySLF30-300 Hz1,000-10,000 kmUltra low frequencyULF300-3000 Hz100-1,000 kmVery low frequencyVLF3-30 kHz10-100 kmLow frequencyLF30-300 kHz1-10 kmMedium frequencyMF300-3000 kHz100-1000 mHigh frequencyHF3-30 MHz10-100 mVery high frequencyVHF30-300 MHz1-10 mUltra high frequencyUHF300-3000MHz10-100 cmSuper high frequencySHF3-30 GHz1-10 cmExtremely high EHF30-300 GHz1-10 mmfrequency
“RF heating,” then, is most broadly defined here as the heating of a material, substance, or mixture by exposure to RF energy. For example, microwave ovens are a well-known example of RF heating. RF heating can have advantages of high speed, deep or adjustable heating depth, or even selective heating where one component of a mixture receives more heat than another. For instance, RF energy may penetrate wood fibers to cure internal adhesives without scorching. In some petroleum processes boiling water is added to relatively cold petroleum ores and the temperature of the resulting mixture/slurry may be insufficient. As it can be expensive to raise the temperature of water above the boiling point at atmospheric pressure RF heating techniques can increase slurry heat without steam or pressure vessels.
The nature and suitability of RF heating depends on several factors. In general, most materials accept electromagnetic waves, but the degree to which RF heating occurs varies widely. RF heating is dependent on the frequency of the electromagnetic energy, intensity of the electromagnetic energy, proximity to the source of the electromagnetic energy, and type of RF energy fields. RF heating material properties include conductivity of the material to be heated, whether the material to be heated is magnetic or non-magnetic, and the dielectric polarizability. Pure hydrocarbon molecules are substantially nonconductive, of low dielectric loss factor and nearly zero magnetic moment. Thus, pure hydrocarbon molecules themselves are only fair susceptors for RF heating, e.g. they may heat only slowly in the presence of RF fields. For example, the dissipation factor D of aviation gasoline may be 0.0001 and distilled water 0.157 at 3 GHz, such that RF fields apply heat 1570 times faster to the water in emulsion to oil.
Similarly, a water/hydrocarbon mixture may not achieve desirable RF heating. Water, even distilled water, can be a susceptor for RF heating. However, the use of water in RF heating is limited to 212° F. (100° C.) at atmospheric pressures, as the vapor phase of water (steam) is a poor RF susceptor. Moreover, in some regions, water resources may be insufficient, and it may be limiting or even impractical to process petroleum ores with hot water or steam.